Efforts are continually being made to increase the printing/copying speed of laser printers and copiers while, at the same time, reducing their cost.
One such laser printer/copier scans one or more laser beams over the surface of a photoreceptor in a repetitive raster fashion, using a rotating polygonal mirror.
In order to increase the speed of operation of such a device, the prior art has attempted to use plural, concurrently scanned, laser beams to achieve an increase of the exposure/print speed. Mochizuki et al. describe such a system in "Dual Beam Diode Laser Scanning System for High Speed Laser Beam Printers" Proceedings of the I, S and T Conference, Japan 1993, pages 222-225. To achieve their dual beam scan system, Mochizuki et al. employ isolated diode lasers whose laser beams are fed through complex optical pathways to a scanning mirror. A rotationally asymmetric aspheric lens is employed to enable use of long wavelength diode lasers. Complex optical pathways and lenses add significant cost to the printer/copier.
In U.S. Pat. No. 5,691,759 to Hanson, assigned to the same Assignee as this application, a laser printer is disclosed which produces a raster image of plural scan lines of image pixels, each raster scan line separated from an adjacent raster scan line by a pitch distance p. The printer includes a movable photoreceptor and n laser sources, where n.gtoreq.2. The n laser sources produce n optical beams that, at the photoreceptor, are separated by a distance of (n+1)p in a direction of movement of the photoreceptor. A scanner scans the plural optical beams in parallel paths across the photoreceptor, as they are modulated in accordance with raster scan line pixel data provided from an image buffer. At the completion of each scan, the photoreceptor is moved by a distance (n.times.p), and the n optical beams are again modulated with pixel values from another set of raster scan lines that are respectively, (n+1)p distant from the raster scan lines of the first scan. In such manner, an interleaving of the scan lines is achieved, using plural optical beams from a single laser diode chip, to produce the n laser beams. The disclosure of U.S. Pat. No. 5,691,759 is incorporated herein by reference.
The required resolution of newly designed laser print engines is now up to 1200 dots per inch (dpi), which equates to inter-dot spacings of about 21.mu.. Currently available low cost laser scan mechanisms cause a 5.times. to 10.times. magnification, at the photoreceptor, of the inter-beam spacings.
When diode lasers are integrated into a single chip structure, if their inter-diode spacings are less than about 50.mu., thermal effects between adjacent diodes begin to affect system performance.
In the system described by Hanson in U.S. Pat. No. 5,691,759, the inter-diode spacing in the process (or paper-movement) direction is fixed by the expression (n+1)p, where n is the number of lasers and p is the required inter-dot pitch distance on the photoreceptor. If 4 lasers are incorporated into a laser chip, as shown in FIG. 8 of Hanson, the required inter-diode spacing is (4+1)p or 104.mu. at p=21.mu. on the photoreceptor (in a system where there is no magnification in the projection path). To produce a resolution level of 21.mu. between dots on a photoreceptor with a 5.times. magnifying scanner, requires that the inter-diode spacings on the laser chip be about 21.mu. (i.e., 104/5), which is less than half the required inter-diode spacing.
Accordingly, it is an object of this invention to provide an improved method and apparatus for employing multiple beams to scan a photoreceptor in a laser imaging device.
It is another object of this invention to provide an improved multi-beam, scanned laser imaging device wherein plural laser diode sources are utilized that are spaced in a manner to avoid inter-diode effects.